Farm Progress is part of the Informa Markets Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Serving: Central

Program screening pigweed for resistance to glyphosate

Since the early April announcement that glyphosate-tolerant pigweed had been found in northeast Arkansas, farmers across the state have wondered if their fields might harbor the same. So have Extension researchers and company reps.

“We've looked at Georgia and other states with tolerant/resistant pigweed to see (how best to approach) this problem in Arkansas,” says Ken Smith, Extension weed scientist based in Monticello, Ark. “Once they find a location of resistance it doesn't seem long before they find more.

“We get calls every year from folks saying, ‘Hey, my pigweeds won't die.’ But most of that is application error. We've checked a lot of fields and only that one northeast area has proven glyphosate-tolerant.”

Still, the calls continue to pour in. So Smith and colleagues have set up a project to screen fields with pigweed escapes. Such escapes aren't hard to find.

“Riding through the state today, we saw several places that were suspect. There were two or three pigweeds in a couple of spots in a 100-acre field. Those are the ones that intrigue me, not the fields where pigweeds are overrunning things. The scattered escapes tend to point away from application mistakes.”

The set-up

Farmers are being encouraged to report any pigweed escapes to their county Extension agents. After doing so, Ryan Doherty may visit. The pigweed screening program is part of Doherty's graduate thesis and his greenhouse work is ready to go.

“What we plan to do is collect seed with the help of county Extension agents, farmers and consultants,” says Doherty. “We want to know of anyone that's saying, ‘I know I sprayed these pigweeds and they didn't die.’

“In reality, when we test some of those pigweeds they're just as susceptible as their cousins. But we feel this approach will give us a better chance of finding any that might be more tolerant. By winter meetings, we hope to have data for the farmers.”

Collected seed will be taken to the greenhouse where they'll germinate and then be sprayed with glyphosate. Planted in a flat adjacent to one containing known susceptible pigweeds, the suspect pigweeds will be sprayed with various rates of glyphosate to determine if they are more, or less, susceptible.


If there are, in fact, more tolerant/resistant pigweed locations in the state, Smith wants to know where and how widespread they are.

“I believe we'll find at least several more locations. Initially, I thought we wouldn't find more. But we now know of other locations with weeds not dying. (Doherty) has sprayed some of those pigweeds with a full use rate and they didn't die. So it seems there are at least more tolerant parents out there. That means this pigweed is likely more widespread than I initially thought.”

Monsanto is helping pay for the screening program. The company's interest is obvious — every time a new weed is found able to handle glyphosate, Roundup Ready technology is devalued a bit more.

Monsanto, says Smith, “has said it will stay in the background and take the data as it's supplied.”

The idea for the screening program came out of the Arkansas Herbicide Resistance Weed Committee. The nine-member committee — consisting of Extension personnel, independent consultants and industry representatives — has already released several brochures on resistance threats.

“There a couple of things to focus on here,” says Zach Shappley, Monsanto technology development representative and committee member.

  • First, continue raising awareness. “That's happened but, as we go forward, we need even more. You know, at county and winter meetings this is brought up and people say, ‘Yeah, I've had that for years.’ But what we need is for people to notice when it's new and say, ‘maybe I need to call and have someone take a look.’ If it turns out there is no problem, great.”

    The program will help expose how widespread the problem is, or isn't. And everyone is interested, says Shappley.

    “I hear it almost every time there's a meeting — ‘Will I have it next year? How far has it progressed? Will I have it soon?’ Everyone wants to know how much this threatens them.”

  • Second, how tolerant are the pigweeds? “Do they just wilt and die? Or are they really resistant where they're sprayed and there's no mark.”

  • Third, if you have weeds that don't die after an application, ask why. Chances are tolerance or resistance isn't the cause. “Are you going too fast? Putting out the right rates? There's a host of possibilities. Roundup Ready Flex provides flexibility and convenience. But the other side of that coin is some folks want to load up their sprayers and go to town. They want to run through fields as fast as possible. Convenience is good, and we want it. But we don't want it at the expense of poor weed control.”

Besides finding problem weeds, say the researchers, the screening program will allow producers to zero in on any potential chinks in their spraying programs.

“It'll be good to know weeds aren't tolerant,” says Smith. “But hopefully it'll give folks a reason to consider why they're seeing misses.”


Smith believes the study will show a very small percentage of pigweeds screened are truly tolerant of glyphosate. Even so, the threat is real.

“We need to think about the susceptible female in the field that escapes. Maybe the application happened too fast and there wasn't good coverage. Whatever the case, she's out there in that field as a receptor for any resistant pollen that might be floating around.”

Cleanliness is very important. Keeping clean fields early postpones resistance much longer.

“Think long-term,” suggest Shappley. “There are many chemistries other than glyphosate that can provide agronomic advantages and maintain (Roundup Ready crop viability).”

Roundup Ready crops have allowed farmers to take on more and more acres. Because of that, “they feel they're pushed to cover as many acres as possible every day,” says Smith. “That has developed a situation where efficiency is being hurt and will cause problems in the long run.”

As for glyphosate-resistant horseweed, Smith has noticed a “tremendous amount” that wasn't treated properly this year.

“We had a great number of people who didn't believe they had glyphosate-resistant horseweed and it got out of control…That gave me a reality check. We've been talking about resistant horseweed a lot but obviously not enough. It's going to take an even bigger educational effort for pigweed because it's much, much more serious than horseweed.

“Horseweed is a serious inconvenience and is costly to control. But pigweed will destroy your crops. Pigweed can drive us from the cotton business. If we don't manage it prior to emergence, the cotton crop will be ruined.”

Smith was recently alarmed during a field visit in Phillips County. The field was sprayed with a full rate of glyphosate and some of the pigweeds “acted like they hadn't been touched. It was a text-book example of a scary field. Everything was clean except for four or five car-sized spots.”

The plants hadn't gone to seed and the farmer was advised to chop the pigweeds out. “He did what we suggested. Will those spots still be there next year? Probably, but at least he'll know where to be watching.”

Farmers with suspect pigweeds should be thinking about harvest now, says Shappley.

“If there are areas in the fields with potential weed problems, they should plan to harvest those fields last, if possible. If not, then clean up the machinery (between fields). It's a pain in the neck, but you need that clean-out process so potential problem seed isn't taken around the farm.”

(For more on the Arkansas tolerant pigweed find, see

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.